Expendable cap for fuse cutouts



Feb. 8, 1966 R. J. BRONIKOWSKI 3,234,349

EXPENDABLE CAP FOR FUSE CUTOUTS Filed July 50, 1963 INVENTOR. Fey/vam 'JBRO/V/KOM/JK/ W4 T Q United States Patent Ofilice 3,234,349 Patented Feb. 8, 1966 3,234,349 EXPENDABLE CAP FOR FUSE CUTOUTS Ray on Bron ko s70 ama ack 1-, South Milwaukee, Wis. Filed July so, 1963, Ser. No.298,792 6 Claims; (Cl'. 200-127 This application is a.continuation-in-part of my. application. Serial No. 86,739 tiled.February. 2, 1961, now abandoned.

This invention relates generally to fuse cutouts. and more particularlyto an improvement in expendable caps therefor.

Contemporaryfuse cutouts utilize a fuse tube which is vented at thelower. end and. which is provided at: the other end with a rupturableenclosing member. Upon low current faults, single. venting of the fusetube takes place. However, on. higher faults up through themaxie murninterrupting capacity of the. cutout, the enclosing member. ruptures asa result of internal fuse. tube pressures acting thereon. Thepressure.increase Within the fuse tube is broughtabout by virtue of the factthatgases are evolved within the confines of 'thetube. at a. muchfasterv rate than they are vented therefrom.

US. Patent 2,702,842 to Jepsen et a1. discloses one of the most commonlyutilizeddesigns for these expendable caps and has provenrelativelysatisfactory as witnessed by its commercial success. However, inattempting to raise the interrupting capacity of a fusecutout of thetype described to approximately double the standardratings, I have foundthat contemporary expendablecaps limit the interrupting capability ofthe cutout to alevel approximately 25% below the desired ratings. Thishas been largely due to the fact that the high pressure gases within thefuse tube, upon blowwoif of thev expendable cap, tended to drift or beblown downwardly along. the fuse tube eventually bridging the tubecontacts and causing fiashover of the tube. Also, these suddenlyreleased gases often were blown or attracted to the grounded hangarbracket, and/o r-parts associated therewith, causing flashoverbetweenthe bracket and current carrying portions of the cutout (line toground). In both cases, the operating performance of the cutout isseriously impaired'and burned or otherwisedamaged cutouts resulted. Dueto the above effects, the effective current that a cutout couldinterrupt was approximately 25% below the desired interrupting rating ofthe device. 1

Ihave found that the patterns assumed by the gases escaping from a fusetube, subsequent to blow off of the expendable cap, are the. main causeof this 2.5% reduction of the interrupting capability of a fuse cutout.In this connection I have designed an improved cap which precludes theformation of the deleterious gas patterns which cause reductions incutout interrupting ability.

It is therefore an object of this invention to provide an improvedexpendable cap which is adaptable to present cutout structures.

Another object of this invention is to provide an improved expendablecap which will function in such a manner that the interruptingcapability of the fuse cutout is not materially reduced as a result ofthe formation of gas patterns which are conductive to flashover.

Another object of the invention is to provide. an improved expendablecap which is economical to produce and which is dependable in operation.

A further object of the invention is to. provide an expendable cap whichwill blow off at values of internal tube pressure below 1000 p.s.i.

A still further object of the invention is to provide an improvedexpendablecap which will blow off at minimum current values of -90% ofthose at which contemporary caps blow.

Other objects and advantages of my invention will be apparent from thefollowing description of the preferred embodiments of the inventiontaken in connection with the accompanying drawings in which:

FIG. 1 is a partially cut away view in elevation of a fuse cutoutincorporating an improved expendable cap.

FIG. 2 is a cutaway. view in elevation of a portion of the fuseholderassembly and expendable cap of FIG. 1.

FIG. 3 is a partially cutaway view in elevation of the expendable capofthe invention.

Referring to FIGURE 1, 10 indicates a fuse cutout generally whichcomprises a skirted insulating member 11 having an upper contactassembly 12 and lower bearing members 13 afiixed thereto. The uppercontact assembly 12. includes. a pivotal latching member 14, resilientcontacts.15.and terminal seat 16. Associated with the bearingmember 13are lower resilient contacts 17 and a lower terminal member 18.

A fuseholder. assembly 19 is journalled in the bearings Band is normallyadapted to electrically bridge the contacts 15 and.17. The assembly 19includes a fuse tube 20 which is capable of evolving quantities ofgases, upper fuse tube contact assembly 21, pull ring assembly 22 andhinged trunnion and contact means 23. The fuse tube contact assembly 21includes a ferrule member 24 which is fixedly attached to one end of thefuse tube 20. and which. has a projecting conducting portion 25, theouter endof which is normally adapted to engage contacts 15. The upperportion of the ferrule member24 has external screw threads 26 thereonwhich serve as a seat for ex.- pendable enclosing means 27 which ispressure sensitive and whichwill be explained in greater detailhereinafter. A rupturable fuse link 2.8 is positioned within the fusetube 20 and is inelectrical engagement with fuse tube contact assembly21 and anchored at its other end to member 23.by means 29.

As can be seen, the fuseholder is normally in contact bridging position.When a fault is experienced; on the line the overcurrent will. servetomelt the fuse link and the resultant arc in the fuse tube will beextinguished as a result of the gases evolved from the fuse tube.Initially, these evolved gases will vent from the bottom of thefuse tubebut as the rate of their evolvement exceeds the rate at which they arevented a pressure build up in the tube occurs. This pressure build up,if sufliciently high, will cause the expendable enclosing (cap) means torupture thereby allowing gases to vent from both ends of the fuse tube.Subsequent to are extinction, as a result of the rupturing of thefuselink, relative movement between the fuse tube and trunnion means 23occurs and the fuseholder assembly thereafter pivots aboutbearin gmembers; 13 and out of contact bridging position.

Referring now to FIGS. 2 and 3, the expendable cap is. indicatedgenerally at 27 and comprises a portion 30:

having internal screw threads 30 which are adapted to engage theexternal screw threads 26 on ferrule member 24. The enclosing portion ofthe cap 27 includes a shoulder portion 31, an inwardly spaced annularrupturable (frangible) portion 32, a first annular projection 33 and acentral raised portion 34, thereby forming a bottom wall on saidcup-shaped cap 27 which increases in thickness progressively in adirection radially inward from said frangible portion 32.

The upper end of the fuse link 28 comprises a reduced fusible portion 35and a bottom head 36 which is normally of a somewhat greater diameterthan the bore of the ferrule member 24 and Which rests upon the upperextremity of the ferrule member. As can be seen the upper surface of thebottom head 36 abuts against the relatively flat inner surface of thecap 27. A fibre liner 37 surrounds the fuse link and fusible portionthereof and extends into the bore of the fuse tube 20 for a distance andhas for its purpose the confining and extinguishing of low current arcs.

In operation, as gases are evolved from the interior of the fuse tube, aforce is exerted by these gases on the underside of the button head anduniformly distributed to the under surface of the cap immediately abovethe button head. The portion 32 is the thinnest portion of the cap andis designed to rupture under a given internal pressure. Upon rupture 'ofthis section of the cap, it and the button head will blow out, therebyallowing gases to be vented from that end of the tube. A large portionof the force exerted by. the button head is transmitted to the firstprojection 33'and central raised portion 34 of the cap. This causesbowingfor bulging of the bottom wall portion of the cap which increasesprogressively in a radially inward direction and thereby causes therupturable portion 32 of the cap to be put in tension. This tensileforce on area 32 is in addition to the shear force exerted on this areawith respect to the shoulder portion 31 of the cap. The particularconfiguration of the cap which results in rupture occurring due totensile and shear forces enables a closer control of the rupturing pointto be obtained as well as permitting rupture at a more optimum internaltube pressure than would otherwise be possible, if only shear forceseffected rupture. Contemporary caps rupture at internal pressures in therange of 1700-2300 p.s.i. which is in accordance with the industrystandards of 2000 p.s.i. plus or minus percent.

This invention basically involves an expendable cap in which therupturable section of the cap is designed to rupture at static burstingpressures of 1000 p.s.i. or less. A fuse cutout having an expendable capin accordance with the invention has a maximum interrupting capacityapproximately on-third higher than the same fuse cutout with aconventional cap. Tests establish that a fuse cutout rated at 100amperes having an expendable cap in accordance with the invention willrepeatedly interrupt 17,000 amperes successfully, Whereas the same fusecutout with a conventional cap cannot clear currents above 12,000amperes repeatedly without flashover. If the pressure rises above 1000p.s.i., gas patterns occur which are conducive to fuse tube flashover orline to ground flashover, both of which may cause considerable damage tothe equipment to be protected, the fuse cutout and associatedstructures. Hence, if the static bursting pressure of the cap is notkept to values of 1000 p.s.i. or less,

Expendable caps in accordance with the invention which are designed torupture (blow off) at internal tube pressures of 1000 p.s.i or less willprovide double fuse tube venting in a shorter time after arcing hascommenced and at minimum currents which are 80 to 90% of thoseencountered when conventional caps rupture.

The frangible portion 32 preferably has a minimum thickness which willprevent rupture thereof at pressures below approximately 600 p.s.i. Suchminimum thickness of frangible section assures that the cap will notrupture in the current range below 1500 amperes wherein clearing of thearc is efiected more eificiently by venting at a single end of the tube.

In this manner, the effective interrupting capability of the cutout isnot reduced by the sudden high pressure gases venting from the tube andsetting up flashover gas patterns, since the pressure at which the gasesare vented is below that at which these deleterious gas patterns form.

While one particular embodiment of the invention has been shown anddescribed, it will be obvious to those skilled in the art that variouschanges and modifications can be made therefrom without departing fromthe invention and, therefore, it is intended for the appended claims tocover all such changes and modifications as fall within the true spiritand scope of the invention.

I claim:

1. In an expulsion fuse cutout, in combination, a fuse tube open at bothends, a metallic ferrule aflixed to one end of said fuse tube, a fusibleelement within said fuse tube, an expendable metallic cap closing saidone end of said fuse tube and having a threaded side wall portionengaging said ferrule and a bottom wall portion extending above theopening in said one end of said fuse tube and-means joiningsaid sideandbottom Wall portions and being rupturable at internal tube. pressurefrom 600 to 1000 p.s.i. to separate said side and bottom wall portionsand vent said one end of said fuse tube, whereby gas patterns whichcause electrical flashover of said cutout are prevented and theinterrupting capacity of said fuse tube is increased.

2. A generally cup-shaped expendable .cap adapted to be affixed to andclose one end of a fuse tube open at both ends, said cap having athreaded continuous sidewall portion and a bottom wall portion adaptedto extend above the opening in said one end of said fuse tube and meansjoining said side and bottom wall portions and being partible toseparate said side and bottom wall portions at internal tube pressuresfrom 600 to 1000 p.s.i. and at pressures below those which areaccompanied by patterns of expelled gases conducive to flashover of saidfuse tube.

'3. A generally cup-shaped expendable cap adapted to be affixed to andenclose one end of a fuse tube, the bottom wall of said cup-shaped caphaving means defining an annular frangible section of such thickness andstrength that it will rupture and the portion of said bottom wallsurrounded by said frangible section will separate from the remainder ofsaid cap when subjected to internal tube pressures from 600 to 1000p.s.i. and below those pres sures which are accompanied by patterns ofexpelled gases conducive to flashover of said fuse tube. V

4. A generally cup-shaped expendable cap adapted to enclose one end of afuse tube, the tubular sidewall portion of said cup-shaped cap beingthreaded to engage said one end of-said fuse tube and the bottom wall ofsaid cupshaped cap having means defining an annular frangible section ofsuch thickness and strength thatit will rupture and the portion of saidbottom wall surrounded by said frangible section will separate from theremainder of said cap when subjected to internal tube pressures from 600to 1000 p.s.i. and below those pressures which are accompanied bypatterns of expelled gasesconducive to flashover of said fuse tube, thethickness of said bottom wall of said cap increasing progressively in adirection radially inward from'said annular frangible section.

5. In an expulsion fuse cutout, in combination, a fuse tube open atbothends and having a material on its inner periphery adapted to evolvearc-extinguishing gas when subjected-to an electric arc, a metallicferrule afiixed to one end of said fuse tube, a fusible element withinsaid fuse tube having a metallic button head at one end disposed againstsaid ferrule, a cup-shaped expendable metallie cap removably secured tosaid ferrule and closing said one end of said fuse tube, the bottom wallof said cupshaped cap being disposed above said button head and havingmeans defining a frangible portion of such thickness and strength thatit will rupture and separate from the remainder of said cap at internalpressures within said fuse tube between 600 and 1000 p.s.i. and whichare below those pressures which produce patterns of expelled gases thatfiashover said fuse cutout.

6. In an expulsion fuse cutout in accordance with claim 5 wherein saidfrangible portion is annular and the bottom Wall of said cap increasesprogressively in thickness References Cited by the Examiner UNITEDSTATES PATENTS 2/1955 Jepson 200-127 8/1958 Wood 200127 10 ROBERT K.SCHAEFER, Acting Primary Examiner.

BERNARD A. GILHEANY, Examiner.

2. A GENERALLY CUP-SHAPED EXPENDABLE CAP ADAPTED TO BE AFFIXED TO ANDCLOSE ONE END OF A FUSE TUBE OPEN AT BOTH ENDS, SAID CAP HAVING ATHREADED CONTINUOUS SIDEWALL PORTION AND A BOTTOM WALL PORTION ADAPTEDTO EXTEND ABOVE THE OPENING IN SAID ONE END OF SAID FUSE TUBE AND MEANSJOINING SAID SIDE AND BOTTOM WALL PORTIONS AND BEING PARTIBLE TOSEPARATE SAID SIDE AND BOTTOM WALL PORTIONS AT INTERNAL TUBE PRESSUREFROM 600 TO 1000 P.S.I. AND AT PRESSURES BELOW THOSE WHICH AREACCOMPANIED BY PATTERNS OF EXPELLED GASES CONDUCIVE TO FLASHOVER OF SAIDFUSE TUBE.